There are known a method in which for the purpose of modifying the surface of a fluoroplastic, which is difficult to adhere because of its small affinity for other substances, a fluoroplastic is immersed in a treatment solution comprising liquid ammonia or naphthalene containing metal sodium and tetrahydrofuran to modify the surface, as well as a method of chemically modifying the surface of a polyethylene or polypropylene resin, which is chemically inactive and therefore difficult to directly print or adhere, by dipping into a mixed solution of potassium dichromate with concentrated sulfuric acid.
Unfortunately, these conventional chemical modification methods have problems in that, in the case of a fluoroplastic, for example, the surface of a fluoroplastic material turns brown to make the surface layer brittle, resulting in peeling of the adhesive layer. Consequently, no satisfactory adhesive strength can be obtained. In addition, although the above conventional methods can modify an entire portion which is immersed, they cannot perform partial modification unless a photoresist is used as a mask. Also, the treatment reaction is difficult to control, and dangerous chemicals must be used.
There are some other known methods, such as sputtering, corona discharge, and plasma processing, by which the surface of a fluoroplastic or the like is physically modified. However, because of the lack of chemical affinity for an adhesive, a roughened surface formed acts as a stress concentration point, bringing about bonding breakdown. This makes it impossible to obtain a high bonding strength.
A polymeric porous membrane or film made from a fluoroplastic, such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride, or from a polyethylene or polypropylene resin is being used as a filtering membrane in precision filtration and ultrafiltration of liquids such as chemicals, foods, and water. Several methods are known as a means for imparting hydrophilicity to these porous membranes in order to improve the rate of permeation of liquids. Examples are a method of coating a surface active agent, and a method as described in Jpn. Pat. Appln. KOKAI Publication No. 56-63772, in which pores of a porous body are impregnated with a water-soluble polymer, such as polyvinyl alcohol or polyethylene glycol, and hydrophilicity is imparted to the porous body by, e.g., a heat treatment, acetal conversion, esterification, a dichromic acid treatment, or irradiation of ionizing radiation. There is another known method by which the surface of a fluoroplastic is modified by giving hydrophilicity to the surface by irradiation of an ArF laser, as disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 5-77692.
In the method of coating a surfactant, however, the surfactant is readily removed since it does not strongly adhere to a porous body, resulting in difficulty in keeping hydrophilicity.
In the method described in Jpn. Pat. Appln. KOKAI Publication No. 56-63772, deterioration by decomposition of a porous body is brought about if, for example, irradiation is used, and this significantly decreases the mechanical strength. In addition, the use of a heat treatment, acetal conversion, or esterification poses a problem of a low degree of hydrophilicity, since a portion of the water-soluble polymer is given hydrophobicity.
Also, the method described in Jpn. Pat. Appln. KOKOKU Publication No. 5-77692 is a method of modification of a surface layer; i.e., it is not possible to sufficiently impart hydrophilicity to the bulk of a porous body.
The present invention has been made in consideration of the above situations and has as its object to provide a method in which a liquid which is safe at room temperature is brought into contact with the surface of a workpiece (a material to be modified), and in this state light such as ultraviolet light is irradiated on the surface of the workpiece to selectively and effectively substitute the surface with an arbitrary functional group with no damage to the surface at all, thereby modifying the surface of the workpiece. The present invention also provides a treatment apparatus for this method.
Preferably, it is an object of the present invention to provide a method in which the liquid described above is forced to closely contact, as an extremely thin layer, with the surface of a workpiece by using a capillary phenomenon or the like, and in this state light such as ultraviolet light is irradiated on the surface of the workpiece to selectively and effectively substitute the surface with an arbitrary functional group with no damage to the surface at all, thereby modifying the surface of the workpiece, and to provide a treatment apparatus for this method.
The use of a liquid as a surface modifier allows a high-density, uniform treatment, since the density of contact with an object to be treated is high compared to a gas. The use of a liquid is also advantageous from the standpoint of environment because only an exposed portion is activated by light such as ultraviolet radiation.
Incidentally, when laser light is incident on a solution of the type discussed above, bubbles are usually generated by photo-decomposition, and consequently a treatment solution on the interface of an object to be treated is removed from the interface. Then, an effective chemical reaction may not take place. Additionally, if the contact angle, with a reaction solution of the surface of an object to be treated is large, the area of contact with the liquid surface becomes small. This contact area is further decreased by generation of bubbles described above.
In the present invention, therefore, the space between the surface of glass as an entrance window for light such as ultraviolet radiation and the surface of a workpiece is made very thin. Consequently, a reaction solution enters this portion to permit formation of a thin liquid film on the surface of a workpiece regardless of the contact angle of the material with the solution. When light such as ultraviolet radiation is irradiated on the surface of the workpiece in this state, not only the liquid but the surface of the workpiece can be sufficiently excited because of a short path in the liquid. This makes effective optical modification feasible.
As discussed above, the surface layer of a solid workpiece can be photochemically, modified effectively by interposing a thin film of a reaction solution between the surface of the solid workpiece and transparent glass by using capillarity, and irradiating light on the surface of the workpiece in this state.